Definitive MPLS Network Designs [Electronic resources] نسخه متنی

Carrier's Carrier Architecture

The initial Layer 3 MPLS VPN service consisted of the architecture detailed in the previous sections and was primarily targeted at providing Layer 3 network-based VPN services to customers with a limited amount of IP routing information (because of their relative size in terms of regional and global reach). With restricted routing information, the service provider PE routers could scale to a large number of end customers. This model is still the primary delivery vehicle for Layer 3 MPLS VPN services today.

However, over time larger customer deployments have been requested, including large Enterprises and Internet service providers (ISPs), as well as other VPN providers, that want to obtain IP-based any-to-any connectivity. This requirement has been driven in part by the desire of these customers not to have to build their own Layer 2 network, but instead to use the Layer 3 MPLS VPN network for transport. With the reduction in infrastructure costs and the added benefit of full any-to-any connectivity, this service is very attractive.

Although the service is attractive for large Enterprises and other carriers for the reasons already noted, it comes at a considerable cost, in terms of scale, to the operator of the Layer 3 MPLS VPN network. For example, consider an ISP as the customer of this service. It might want to exchange full Internet routes (some 155,000+ routes) between its sites. Because the Layer 3 MPLS VPN service requires that customer routing state be stored at the PE routers and be distributed to other PE routers within its control, this presents a significant scaling problem at the edge of the network, not to mention the route distribution load offered to the control plane within the service provider network.

As a trade-off to provide connectivity for customers with large routing requirements, and as a complement to the regular Layer 3 MPLS VPN services for other VPN or non-VPN carriers that want to obtain basic IP connectivity, the Carrier's Carrier architecture was introduced. This architecture allows for the direct exchange of customer routes between their own sites and the use of the Layer 3 MPLS VPN backbone as pure IP "transport" between these sites. Figure 1-12 illustrates this concept.